Drilling muds are used in the process of drilling bore holes in subterranean deposits for gas and/or oil production. The boring is accomplished by well drilling tools and a drilling fluid. Drilling muds serve to cool and lubricate the drill bits, to carry the cuttings to the surface as the drilling fluid is circulated in and out of the well, to support at least part of the weight of the drilling pipe and drill bit, to provide a hydrostatic pressure head to prevent caving in of the walls of the well bore, to deposit on the surface of the well bore a filter cake which acts as a thin, semi-pervious layer to prevent undue passage therethrough of drilling muds, and to perform other functions as are well-known in the drilling art. It is important that the drilling fluid exhibit a relatively low rate of filtration or fluid loss in addition to having desirable rheological properties, such as viscosity and gel strength.
Drilling muds contain additives and conditioning agents that are important in determining the fluid loss properties of the drilling fluid, as well as inhibiting shale and clay disintegration. U.S. Pat. No. 5,909,779 discloses that such additives or agents include modified lignite, polymers, oxidized asphalt, gilsonite, humates prepared by reacting humic acid with amide or polyalkyl polyamines. The amount of fluid loss agent added to the drilling mud composition is usually less than 10% by weight, and preferably, less than 5% by weight of the drilling mud.
U.S. Pat. No. 5,883,054 discloses adding a random styrene-butadiene (SBR) copolymer having an average molecular weight greater than about 500,000 g/mol to an oil-based drilling fluid to make a thermally stable drilling fluid system. The concentration of SBR is about 1 to about 6 pounds per barrel. According to the patent, the resultant drilling mud system exhibits fluid loss control at high temperatures and high pressure conditions. The reference compared the fluid loss (mL/30 min) of a block styrene-butadiene copolymer (30 wt. % polystyrene) to a random SBR copolymer. The fluid loss property of the random SBR is disclosed to be significantly better than a styrenic block copolymer.
U.S. Pat. No. 5,925,182 discloses adding oil-soluble block or random copolymers to water-based drilling muds. The copolymers comprise styrene-isoprene and styrene-butadiene and can be present in the fluid in an amount ranging from about 0.1 to about 10 wt. %. The copolymer provides a stable liquid composition for use in water-based drilling fluid. However, the reference does not disclose that radial block copolymers of styrene and butadiene significantly reduce fluid loss in drilling muds.
U.S. Pat. No. 6,034,037 discloses a synthetic oil-based drilling fluid containing up to 20 pounds per barrel of a polymeric fluid loss control agent comprising a polymer consisting of at least two monomers selected from the group consisting of styrene, butadiene, and isoprene. Table A discloses a typical drilling fluid containing synthetic oil, lime, fluid loss control agent, organophilic clay, brine, weighting agent, and a styrene-butadiene copolymer.
U.S. Pat. No. 6,017,854 discloses a non-aqueous drilling fluid containing styrenic block copolymers to help prevent fluid loss. As disclosed in the U.S. Pat. No. 6,017,854, selectively hydrogenated isoprene-styrene block copolymers are employed as a fluid loss agent for a low toxicity synthetic drilling fluid. The examples of the patent disclose use of a linear styrene-ethylene/propylene (SEP) comprising 28% styrene and 72% ethylene/propylene. The total block copolymer concentration in the drilling mud is from about 0.01 to 10 wt. %.
WO 2004/022668 claims an oil-based mud containing up to 10 wt. % of a radial styrene-butadiene-styrene copolymer having 25 wt. % or more polystyrene to improve the fluid loss of the drilling mud by decreasing the high temperature, high pressure fluid loss value. Also disclosed within the specification is styrene-ethylene/propylene copolymer (a selectively hydrogenated isoprene-styrene block copolymer).
We have discovered that a selectively hydrogenated isoprene-styrene block copolymer containing at least 40 wt. % or more polystyrene, improves the fluid loss property of a drilling fluid when compared to a drilling fluid that does not contain the copolymer.